Friction of stable gradient nano-grained metals

In this paper, a novel strategy for lowering the coefficients of friction (COFs) of metals during dry sliding is introduced by producing a gradient nano-grained (GNG) surface layer without changing chemical compositions or phase constitution. Surface morphology variation, subsurface structural evolution and stress field analysis unfold that sliding-induced strain localization and microstructural instabilities are significantly mitigated through dislocation activity and grain boundary activities in the GNG surface layers. In addition, design guidelines for GNG materials with unprecedented low COFs are discussed based on interlinking tribological stress, surface deformation mechanisms and friction behavior. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.